1. Field of the Invention
The present invention relates to a non-magnetic toner for use in developing electrostatic images, a developer using the toner, a toner container and an image forming apparatus.
2. Description of the Related Art
As a method for fixing toner, a contact heat fixing method such as a heat roller fixing method has hitherto been widely adopted. A fixing apparatus used in the heat roller fixing method is equipped with a heat roller and a pressure roller, and by making a recording sheet carrying a toner image pass through a pressure contact section (nip section) between the heat roller and the pressure roller, the fixing apparatus melts the toner image to be fixed on the recording sheet.
In the contact heat fixing method typified by the heat roller fixing method, fixing is carried out by bringing the surface of a heating member (for example, a heat roller) of a contact heat fixing apparatus into contact with a toner image on a recording sheet, and hence sometimes there occurs an offset phenomenon that a portion of the toner image is partially adhered to the heating member, and the adhered toner image is transferred to a successive recording sheet to stain the successive recording sheet. Accordingly, in the contact heat fixing method typified by the heat roller fixing method, it is necessary to prevent the above-described offset phenomenon.
There has been known techniques in which, for the purpose of preventing the offset phenomenon, a fixing oil such as silicone oil is applied to or impregnated into the heat roller or the pressure roller of a fixing apparatus. However, from the viewpoint of downsizing of fixing apparatuses and cost reduction, oilless fixing apparatuses in which a fixing oil imparting mechanism is omitted and fixing apparatuses of the types that reduce oil application amounts have been adopted. When such fixing apparatuses are adopted, the toner is added with a releasing agent as offset preventing agent.
Additionally, in the case of the heat fixing methods, the heating temperature is preferably as low as possible for the purpose of energy saving; however, if the thermal properties of the binder resin included in the toner is designed to be compatible with too low temperatures in order to attain low-temperature fixability, the heat-resistance/storage stability of the toner is degraded, and there occur problems such as blocking. For the purpose of making the low temperature fixability and the heat-resistance/storage stability compatible with each other, it is advantageous to use polyester resin as the binder resin. Polyester resins have lower viscosity and higher elasticity than vinyl copolymer resins and hence offer excellent low temperature fixability and heat-resistance/storage stability.
Recently, there have been known polymerization methods and methods for producing new toners, namely, chemical toners so as to replace conventional pulverization methods; among these is a so-called dissolution suspension method as a toner production method that is suitable for using polyester resin. When this method is used, the circularity of the toner tends to be high so as for the toner to be approximately spherical, and the toner can also be made nonspherical.
As is generally known, the toner shape closer to sphere is advantageous in the sense that its transferability is improved and accordingly the image quality is also improved. However, it is difficult to transfer all of such spherical toner particles, and toner particles remaining on the photoconductor and other members need to be removed. However, the conventional blade cleaning method suffers from a disadvantage that spherical toners tend to slip through the blade to cause problems. Accordingly, there have been devised methods in which, without adopting daring application of blade cleaning to spherical toners, toners are recovered in the developing unit or the like, or dispersed with brushes or the like (referred to as a so-called cleanerless system). The use of such a system provides advantageous effects, including smaller image forming apparatus accomplished by absence of toner box or the like, and reduced waste toner amount. Additionally, for the purpose of using such a system, it is important to reduce the amount of the remaining untransferred toner as much as possible, and for that purpose, it is essential to increase the sphericity of the toner so as to improve its transferability.
Additionally, when a toner containing a sufficient amount of releasing agent is produced by using the above-described solution suspension method, it is often the case that the releasing agent is exposed to the toner surface and, in some cases, stains the developing members. The releasing agent can also be embedded deeply in the interior of the toner; however, when the releasing agent is too deeply embedded, bleeding of the releasing agent upon fixation becomes insufficient, leading to generation of offset.
Additionally, Japanese Patent Application Laid-Open (JP-A) No. 2006-227592 discloses a technique for providing a toner having stable charge characteristics even when the dot reproducibility, developability and transferability are improved by reducing the particle size and by making the particles spherical, by controlling the temperature of the medium used in the cleaning step to a specified range.
Additionally, JP-A No. 2006-113553 describes an invention on an image forming apparatus capable of obtaining images of high quality. This invention is directed to an image forming apparatus including a developing apparatus and a cleaning member that removes the substances remaining on the surface of the image bearing member after the toner image has been transferred onto the recording sheet, wherein the above-described developing apparatus contains toner base particles including a binder resin and a colorant, and contains resin particles in which fine particles of a compound having an acid-accepting effect are included in such condition that the fine particles of the compound are partially exposed to the surface of the resin particles.
JP-A No. 2005-49858 discloses an invention on a resin particle for use in a toner, satisfactory in blade cleaning property and excellent in low temperature fixability and anti-hot offset property.
Japanese Patent Application Laid-Open (JP-A) 2003-515795 describes an invention on compounds useful as a charge adjusting agent for a toner for use in electrophotography.